It forces you to make some assumptions on market returns and such, but it gives a pretty clear picture. The biggest variable is how long you expect to live in the same place (longer favors buy) and the next biggest is the ratio of average rent to average housing payment. The inflection point being that if you live in one place long enough to pay off the mortgage, then it obviously starts to be much more advantageous to buy, but that requires you predicting your life 30 years in the future.